id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30493	GenericRelation and prefetch_related: wrong caching with cyclic prefetching.	Finn Stutzenstein	Can Sarıgöl	"Hello @all!

I encountered an issue with GenericRelations. Here is an example to reproduce this issue: https://github.com/FinnStutzenstein/GenericRelatedPrefetch
Just do a migrate and runserver. The code showing the error is started automatically in main/apps.py. Please start with --noreload not to have the output twice.

Whats the problem?
I have a generic model (Tag) that have a {{{content_object}}}. Then there are multiple (in the example 2) models, Book and CD, that have exactly one tag assigned. In the real application (OpenSlides) this invariant is ensured in other places; in the example the objects are created in a way, that this invariant holds.

All these content objects have a property {{{tag}}}, that should return the one assigned tag. This is done by adding a helper field {{{tags=GenericRelation(Tag)}}} and the property accesses {{{self.tags.all()[0]}}}. The {{{.all()[0]}}} instead of a simple {{{.get()}}} is required for the prefetching to work. See main/models.py in the example.

Now all tags should be loaded because in OpenSlides they would be serialized. For each tag the {{{content_object}}} is accessed as well as {{{content_object.tag}}}. Because this would result in many DB queries (in the real application about 10000 Tags, and in sum 10000 content objects) the models are prefetched with: {{{Tag.objects.prefetch_related(""content_object"", ""content_object__tag"")}}} (Note: The executed code is in main/apps.py). This results in a constant amount of queries (4 in this case) instead of something proportional to the amount of objects. In the example you can set {{{N}}}, the amount of objects created, to a higher amount to verify, that the amount of queries stays constant.

What is expected: If I have a tag {{{tag}}}, {{{tag.content_object.tag}}} should be equal to {{{tag}}}.
Output from the example (with N=1):

{{{
Got 'Tag to book0':
    -the content object: Book0
    -the content objects tag (should be the same as 'Tag to book0'!):Tag to book0
Got 'Tag to cd0':
    -the content object: CD0
    -the content objects tag (should be the same as 'Tag to cd0'!):Tag to book0
}}}

This is not the case: 'Tag to cd1' -> 'cd1' -> 'Tag to book1'.

I tracked this a bit showing, that {{{_prefetched_objects_cache}}} holds the wrong value, which is accessed through {{{.all()}}} -> {{{.get_queryset()}}} where the cached/prefetched result is taken.

Thanks!"	Bug	closed	Database layer (models, ORM)	dev	Normal	fixed	GenericRelation prefetch_related		Accepted	1	0	0	0	0	0